Fuel tanks of thermoplastic materials are in principle to be impermeable in relation to hydrocarbons. They are usually produced from multi-layer co-extrudates which include barrier layers for hydrocarbons. The plastic materials used for the production of plastic fuel tanks, for example polyethylene, are in principle not diffusion-tight in relation to hydrocarbons. In addition those plastic materials have a tendency to swell in the presence of fuel. For that reason the known fuel tanks of plastic material are provided with a multi-layer wall in which an EVOH layer is embedded as a barrier to hydrocarbons. In that respect the lack of diffusion-tightness of the base material used does not represent a problem.
Usually however fuel tanks are provided with installation fitments which have air intake and venting conduits as well as electrical connections. To produce passage means which pass through the tank wall, it is known for the finished tank to be provided with marked-out circles into which connecting nipples, valves or the like are fitted. The valves, valve covers and nipples are welded to the outside wall of the tank in such a way as to cover the respective opening. Such connecting locations on a plastic tank represent potential leakage points. In order to avoid leakage paths at connections of the fuel tank it is known for connecting nipples and valves to be produced in the form of complicated and expensive two-component structural members, wherein generally those structural members have at least one part comprising a plastic material which is weldable to the tank wall, whereas functional parts on such members usually comprise plastic materials which do not swell in the presence of fuel. Those plastic materials (ABS, POM) generally cannot be welded to the plastic materials used for the tank wall.
Frequently installation fitments, valves and connecting nipples have to be connected to the tank in a manual manufacturing stage after the tank is finished. It is known for example for parts of valves to be latched on the inside of the tank wall and to be connected in positively locking relationship on the outside to weld-on covers, weld-on nipples or the like. The complication and expenditure involved in assembly is considerable.
A process for the production of a fuel tank of thermoplastic material and a fuel tank with connecting elements for hoses or the like is known from EP 1 211 196 A1. EP 1 211 196 A1 describes a dual-shell fuel tank of thermoplastic material with a barrier layer for hydrocarbons, which is embedded in the tank wall and whose shell portions which are assembled to form the tank were produced by thermoforming. In order to ensure ease of mounting connecting elements, while maintaining a high level of permeation sealing integrity for hydrocarbons, EP 1 211 196 A1 proposes a two-part connecting element or a two-part fitting which includes at least an outer portion and an inner portion, wherein the outer portion is welded to the outside wall of the container and the inner portion at least partially passes through the outer portion, with displacement of the container wall and intermediate positioning thereof. That arrangement has the advantage that the container wall is enclosed and displaced between the component parts in such a way that the displaced material serves as a sealing means for the passage assembly passing through the tank wall.
Such a passage means passing through the tank wall can be comparatively easily produced when thermoforming the half-shell portions of the tank, but in contrast such a connection is more difficult to implement when producing the fuel tank by extrusion blow molding.
Therefore the object of the invention is to provide a process for producing a container, in particular a fuel tank of thermoplastic material, with which it is possible to provide connecting elements thereon during the production of the container, with means of the utmost simplicity. In that respect the invention seeks to provide that leakage paths for hydrocarbons in the wall of the container are as far as possible to be avoided or restricted to a minimum.